Short Chain Fatty Acid Production

Principal end products of bacterial fermentation in the colon are SCFA, i.e., acetate, propionate, and butyrate. Other fermentation products include ethanol, lactate, succinate, formate, valerate, and caproate. Branched chain fatty acids such as isobutyrate, 2-methyl-butyrate, and isovalerate may also be formed from the fermentation of amino acids.

Short Chain Fatty Acids

The production of SCFA by the intestinal microbiota serves to salvage energy from the digesta that would otherwise be lost for the host (90). Butyrate provides an important energy source for the intestinal epithelium. Propionate is metabolized in the liver where it possibly serves as a precursor for gluconeogenesis. Acetate is mainly taken up by muscle tissue but is also used by adipocytes for lipogenesis. Lactate is also metabolized by muscle tissue. However, despite the fact that enterocytes only slowly absorb lactate, it is usually found only at low concentrations in the digesta as it is used to a large extent by members of the intestinal microbiota (91) and only accumulates in disease (92).

Probiotics and Short Chain Fatty Acids

Probiotics will, when they are metabolically active, produce organic acids in the intestine; these will mainly be lactate and acetate. Furthermore, the metabolic activity will influence the metabolism of other microbes present in the intestine, through competition for nutrients and through the production of metabolites. It is, however, not really known to what extent probiotics are metabolically active in the human intestine, in particular in the colon, and whether probiotics produce antimicrobials such as bacteriocins in situ. Studies in mice, colonized with a human microbiota, do however indicate metabolic activity (93).

Assessment of the data presented in Table 6 indicates that most probiotics tested do not affect the composition of the fecal short chain fatty acid composition. This may be explained by the lack of metabolic activity of the probiotics in the colon, but it is more likely to reflect the efficient absorption of fatty acids by the colon (2). Therefore, to assess the influence of probiotics, and for that matter also prebiotics, on the availability of SCFA,

Table 6 Influence of Probiotics on Fecal Short Chain Fatty Acids (SCFA) and Fecal Enzyme Activity in Humans, Selected References
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